The present invention relates to a device and a method for coating an object at a high temperature. Such methods are used for the coating of objects, especially gas-turbine blades, for example with heat-insulating layers, anti-corrosion coatings and/or high-temperature superconducting layers.
Nowadays plasma-supported vacuum coating methods are used to coat objects with thin coatings. Amongst these methods is, for example, cathode sputtering which belongs to the group of PVD methods and as a result of its advantages, especially in comparison with the technically older thermal vapour deposition, has in the meantime found very wide application.
According to prior art, in most cases the coating of a substrate is carried out at room temperature or at only a slightly raised temperature. However there are fields of application in which, in order to form the desired layer structure, the temperature of the object to be coated (substrate) has to be very high during coating, for example 700xc2x0 C. to 1100xc2x0 C. This is the case, for example, in the production of high-temperature superconducting layers or anti-corrosion coatings and heat-insulating layers for gas-turbine components, especially turbine blades.
Various devices are known for coating substrates at a high temperature. From Mxc3xcller et al, Praktische Oberflxc3xa4chentechnik (=Practical Surface Technology) Vieweg and Sohn Verlagsgesellschaft, Braunschweig, 1995, page 419, is known for example a magnetron sputter source, and from WO 98/13531 is known a gas-flow sputter source, in which the high temperature of the substrate is generated by means of an electrical resistance heater. This heater is preferably attached to the side of the substrate which is remote from the sputter source, in order to transfer the heat generated by it backwards onto the substrate by means of heat conduction or heat radiation.
Both of the sputter sources represented in these publications are high-speed sputter sources which up to now have also basically been operated with as low a target temperature as possible even in the case of a high substrate temperature. This is described for example in Kienel G. et al, Vakuumbeschichtung (=Vacuum coating) Volume 2, page 160 to 181. Therefore in the case of these conventional high-speed sputter sources the target is cooled. The prior art therefore usually naturally proceeds from water-cooling of the target.
The devices described in the above-mentioned documents have some serious disadvantages. For as a result of the heat radiation which grows very quickly with the substrate temperature and which is proportional to the fourth power of the absolute temperature, considerable energy losses occur as a result of radiation from the substrate towards the walls of the vacuum chamber and towards the sputter source. For this reason, the heater has to be designed for very high power and consumes correspondingly a great deal of energy. On the other hand intensive cooling, such as described above, has to be provided at the chamber wall and at the sputter target, which leads to high consumption of cooling water.
Efficient cooling of the sputter target (sputtering cathode) which is heated both by the gas discharge and by the hot substrate, presupposes very expensive manufacture (bonding) and requires considerable time outlay for the change of target when the target is consumed.
As a further disadvantage must be mentioned that, within the substrate, as a result of its limited thermal conductivity, there is uneven temperature distribution, such that no uniform coating properties can be guaranteed.
The object of the present invention, therefore, is to make available a device and a method for coating an object at a high temperature, in which the substrate can be heated simply, effectively and uniformly.
This object is accomplished by the device according to claim 1, the method according to claim 23 and by uses of this device and this method according to claims 35 to 39. Advantageous developments of the device according to the invention, of the method according to the invention and of the uses according to the invention are given in the dependent claims.
According to the invention, there is located in the interior of the vacuum chamber a coating chamber (inner chamber), the walls of which are formed from a heat-resistant material, preferably with low thermal conductivity, such as graphite or ceramics for example, or from a plurality of metal sheets with high reflecting power in the spectral range of the heat radiation of the target, and thus have a very high heat-insulating effect. The substrate (the object to be coated) and the sputter source (sputtering cathode) are entirely located inside this coating chamber. Advantageously, the walls of the inner chamber can comprise completely or also just in sections, an arrangement of a plurality of parallel metal sheets which are not in thermal contact with one another, or only slightly so. These can be, for example, flat sheets stapled with spacer pieces or also corrugated or embossed sheets. If corrugated or embossed sheets are used, no spacer pieces are required, since here the corrugation or the impressed nubs ensure the necessary spacing between adjacent sheets. The essential contribution to heat insulation is here provided by the vacuum or the rarefied gas between the metal sheets due to its low thermal conductivity. The walls of the inner chamber therefore comprise at least two metal sheets which touch one another at the most in places.
Furthermore, the coating chamber has openings to admit gas and for the extraction of gas, in order to feed in and extract the plasma-generating noble gas (for example argon) or other gases, for example during reactive sputtering.
A device of this type according to the invention does not require any expensive high-temperature heater to heat the substrate. For the operating conditions of the sputter source can be so selected that the target reaches a stationary temperature which is above, for example 50 K above, the desired substrate temperature. Thus the target (sputtering cathode) radiates heat radiation either directly or indirectly onto the substrate and thus heats up the substrate. Thus there is a reverse heat distribution and heat flow to that of the conventional high-speed sputtering methods. For in the latter the substrate is warmer than the cooled target and heats the latter upxe2x80x94in an undesired mannerxe2x80x94whilst in the present invention the target is used as a heater for the substrate.
What is advantageous about this is that consequently no expensive high-temperature heater for the substrate is required. Furthermore, the energy consumption of the whole plant is significantly lower, since the heat losses to the wall of the vacuum chamber and to the sputter source are greatly reduced or disappear completely and moreover the required heating energy is obtained completely from the waste heat of the coating source.
Since the coating source now does not have to be cooled, the target""s requirement of coolant is also significantly smaller. In particular no direct cooling is necessary any more for the sputter source. This considerably simplifies the entire construction, since on the one hand a uniformly good heat transition between target and cooling system is technically complicated and moreover the cooling system present at cathode potential disappears completely. Thus the electrically insulated water ducts for vacuum/air and the water-resistance to the potential adaptation also disappear.
As the sputtering cathode does not have to be cooled independently, manufacturing the sputtering cathode is also possible without bonding plates or bonding. Thus a target change can take place very quickly, reliably and simply.
In the case of a gas-flow sputter source, moreover, no additional shields are necessary on the outer sides of the target.
What is furthermore advantageous about the method according to the invention and the device according to the invention is that the substrate is heated very uniformly and from the side which is to be coated, and consequently uniform layer properties can also be achieved.
Advantageously, the operating conditions of the sputter source are so selected that the target reaches a stationary temperature which is at least 50 K above the desired substrate temperature. The sputter source can here be so designed that a sufficiently large proportion of the target surface passes its heat radiation predominantly directly, e.g. in the case of a magnetron sputter source, to the substrate or indirectly (e.g. in the case of a gas-flow sputter source) via reflection or absorption on the inner walls of the coating chamber.
The sputter source is advantageously so constructed that at this high temperature no negative changes occur which would impair the stable operation of the source. This means in particular the use of sufficiently heat-resistant materials and taking their thermal expansion into account. In the case of a magnetron sputter source, sufficient cooling of the magnets has to be ensured.
In the method according to the invention and the device according to the invention, the target temperature is adjusted according to a balance between a specific electrical power input into the gas discharge at the target on the one hand, and a specific heat extraction from the coating chamber on the other hand, for example by a defined heat-resistance of the coating chamber walls or of the target holder or of a heat-collecting shield optionally disposed in the vicinity of the target, via this shield""s dimensions, position, orientation in the coating chamber and relative to the target or the substrate, and via its cooling power (shielding temperature) control of the heat extraction and thus of the substrate temperature is possible. The heat can then be passed on outside the coating chamber to a cooling fluid (e.g. water) or a cooling gas (e.g. air) via a heat exchange process. In the case where the substrate temperature has to have the high end temperature at the beginning of the coating process already, the sputter source is advantageously operated initially under conditions in which it only acts as a heating source but not as a coating source. This can come about for example by the working pressure in the vacuum chamber being so far increased that the mean free path of the sputtered particles is clearly shorter than the distance between the target and the substrate. Alternatively, between the target and the substrate in the initial phase a swivelling screen can also be swivelled in such that the direct rectilinear connection between sputtering cathode and substrate is interrupted. In the case of a gas-flow sputter source, the gas flow through the sputter source can also be reduced or switched off.
Particularly stable operation and uniform coatings can be realised if the temperature of the object to be coated is determined by means of a heat sensor, for example a thermocouple which is arranged in the vicinity of the substrate in the inner chamber. In this case, the substrate temperature can be controlled exactly via the power input in the sputter source.
Furthermore, bias voltage can be applied to the substrate in order to control exactly the properties of the coating deposited on the substrate.
The substrate temperature in the present method is in the range between 200 and 1500xc2x0 C. The device according to the invention and the method according to the invention are particularly suitable for the following sputtering methods or sputter sources: gas-flow sputtering, hollow cathode sputtering, magnetron sputtering, diode sputtering or triode sputtering.
Thus substrates can be provided with heat-insulating layers, anti-corrosion coatings and/or high temperature conductive coatings, for example formed from refractory metals and/or compounds, e.g. oxides, nitrides, carbides or borides, for example zircon oxide, cerium oxide, MCrAlY alloys where M stands for a metal, or also YBaCuO.
The method according to the invention and the device according to the invention are particularly suitable for coating gas-turbine blades with heat-insulating layers or anti-corrosion coatings.